This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
Conventionally, MRI systems can be broadly assigned to two main categories: 1) “open” systems and 2) “cylindrical bore” systems. In brief, the cylindrical systems are based on a tube like structure, with the patient inserted into the tube, with the imaging gradients constructed on the cylindrical surface such that they closely surround the patient, only leaving open the front and back of the system, FIG. 1A. FIGS. 1a-1c show MRI scanner designs with the patient's long axis represented by the solid arrow and the main magnetic field direction represented by the open arrow. FIG. 1a shows cylindrical magnet, with the patient positioned within the cylinder. Note that patient's long axis and main magnetic field are parallel to each other. FIG. 1b shows an open system, with the north pole and south pole of the magnet above and below the patient (or equivalently, side to side), with the patient laying horizontally and the field directed vertically (i.e. patient's long axis and magnetic field are orthogonal). FIG. 1c shows the hybrid system, with the vertical (or equivalently, side to side) magnetic field of the open system and with the patient positioned within a cylindrical structure such that the patient's long axis is orthogonal to the main magnetic field. The imaging gradients are constructed on the cylindrical surface. Conversely, the open systems often employ a vertically oriented magnetic field, with the imaging gradients built into the structures above and below the patient table, leaving the sides of the system open in that there are no structures to either side or to the front or back, allowing open patient access, FIG. 1b. The location of the gradient coils in each of these structures affects their relative performance, with the open systems suffering from generally poor performance gradients, while the gradients in the cylindrical systems are generally more efficient. The gradient performance affects two aspects of the images obtained, with the high performance gradients allowing images to be obtained faster and with reduced artifacts. Additionally, the relative orientation of the patient to the main magnetic field also affects image quality: 1) the open systems orient the patient orthogonally to the main magnetic field (FIG. 1b), which allows for a more efficient design of the receiver coil system, effectively increasing the SNR; whereas 2) the cylindrical systems orient the patient's long axis parallel to the main magnetic field (FIG. 1a), which compromises the design of the receiver coil, effectively reducing the SNR compared to that which is optimally obtainable from a system with a matching magnetic field strength.
Consider the configuration of a conventional horizontal-bore, cylindrical design MRI system in FIGS. 2a-2c. FIGS. 2a-2c show the position and form of gradient and receiver coils relative to the patient for conventional cylindrical MRI system. FIG. 2a panel shows the gradient coils with windings located on the curved surface of a cylinder, while the receiver coils (gray shading) are drawn as circular coils positioned immediately above and below the patient. FIG. 2b shows the gradient coils wound on the curved cylindrical surface, efficiently encompassing the volume around the patient, while FIG. 2c shows that the receiver coils only have windings above and below the patient volume, leading to a low winding density over the patient body region of interest. The gradient coils are wound on the curved surface of a cylindrical former that completely surrounds the patient. This geometry produces a relatively high density of coil windings around the patient, while encompassing the relatively small volume (i.e. there is typically very little free space between the patient and the coil windings). This accomplished two design advantages, 1) the high density of coil windings can generate high magnetic gradients with easily achievable current levels, and 2) the small volume enclosed by the coils keeps the coils' inductances low, which facilitates rapid switching of the gradients. The main disadvantageous feature of this design is the inefficiency with which signals are received. In FIGS. 2a-2c, the receiver coil is represented as two circular coils, one positioned above and one below the supine patient. This can be thought of as two coils, one on each face of the flat poles of a cylinder, the long axis of which traverses through the patient (i.e. orthogonal to the patient's long axis). In reality, the top and lower coils may be curved slightly to better conform to the patient's body, and there are typically more than two coils. However, the design principle is similar, in that the coils are essentially limited to two surfaces, producing a low density of coil windings over the body region of interest. This low density of windings in the receiver coil results in low sensitivity to signal.
When considering the conventional open MRI system, with a vertical field relative to the horizontal patient (FIGS. 3a-3c), the relative density of gradient and receiver coil windings are reversed. FIGS. 3a-3c show the position and form of gradient and receiver coils relative to patient for conventional open MRI system. FIG. 3a shows the gradient coil with windings located on flat surfaces of the magnet pole pieces, while the receiver coil (gray shading) is drawn as a cylindrical coil fully encompassing the patient body region of interest (the thorax and abdomen in this example). FIG. 3b shows the gradient coils wound on the flat ends of the magnetic pole pieces, encompassing the volume around the patient with a relatively sparse winding density over the patient region, while FIG. 3c shows that the receiver coil only has windings wound on the curved cylindrical surface which efficiently surrounds the patient volume with a high density of windings. In the vertical field magnet the gradient coils are wound on the flat surfaces (represented as a cylinder in this case) producing a sparse winding density over the body region, and consequently the gradients are weak for typical currents that are available. Additionally, the volume enclosed by the (imaginary) cylinder that encompasses the patient region occupies a large volume, leading to high inductance for each coil, making the gradients slow to switch. These two conditions lead to relatively poor performance specifications for these imaging systems. In contrast, the receiver coil can be efficiently wound on the cylindrical surface of a cylinder (encompassing a patient body region such as the thorax), yielding a high winding density over the body region of interest, with consequent high sensitivity to signal reception.
Thus, it can be appreciated that the configuration of the cylindrical and open systems compromises either the design of the gradient system or the signal receiver system. The present invention (Hybrid Vertical-Horizontal MRI System) combines the advantageous aspects of these two designs.